The present invention relates to a method for generating modem transmission signals with quadrature amplitude modulation (QAM) in which scanned values of elementary bandpass signals are formed in digitally coded form, the elementary bandpass signals are weighted with coefficients depending on the information to be transmitted and the modem transmission signals are formed by superposition of the thus weighted elementary bandpass signals.
In an article by H. Schenk, "Entwurf optimaler Sendesignale fur digitale Datensender bei beliebigen linearen Modulationsformen" [A new design method of digital data transmission signals for available linear modulation forms], AEU [Archiv fur Elektronik und Ubertragungstechnik, Electronics and Communication] Volume 31, No. 7/8, 1977, pages 261-266, there is described the manner in which the transmission signals of modems with linear modulation are derived in a simple manner from two elementary signals without the use of filters and modulators. In this article it is presumed that the two elementary signals are perpendicular to one another and the remaining elementary signals can be derived by superposing these orthogonal signals. This superposition generally includes at least one addition and one multiplication. For example, for an 8-PSK (Phase Shift Keying) system according to the prior art as shown in FIG. 1a wherein the elementary bandpass signals e.sub.0 and e.sub.90 are stored, the elementary signal e.sub.45 is found as shown below ##EQU1## In equation (1) e.sub.0 is the elementary signal at .phi.=0.degree., e.sub.90 is the elementary signal at .phi.=90.degree., and e.sub.45 is the elementary signal at .phi.=45.degree..
The derivation of, for example, the elementary signal e.sub.45 thus requires an addition and a multiplication which can be realized only with additional technical expenditures and moreover, the speed of the circuit is reduced with this measure.
Additionally, if in the above example the factor 1/.sqroot.2 and the signals e.sub.0 and e.sub.90 are each available with a word length of 8 bits and the digital/analog converter at the output of the modem transitter is also designed for 8 bits, then e.sub.45 must be rounded off to 8 bits, producing additional rounding errors.